Checking if view is alive before returning it.
Make sure we don't leave view boxes alive by doing a 'bridge' for the on the plot items. Also added warnings if proper cleanup wasn't done.
corrected SVG test, moved to its proper home
Cleanup / fixes:
  - Corrected isQObjectAlive for PyQt and older PySide
  - Warning messages are opt-in using pg.setConfigOptions(crashWarning=True)
Fixes to make crashing less likely on PySide

Merge branch 'clear_cycles' into develop
GraphicsItem.getViewWidget() returns None if widget has been deleted by Qt
Merge remote-tracking branch 'fabioz/fix_widget_dead' into develop